1. Field of the Invention
The invention relates to presses for hydrostatic extrusion.
2. The Prior Art
The present invention relates to a press for hydrostatic extrusion of the kind comprising a press stand in which there is arranged a pressure chamber formed from a high-pressure cylinder with an inner tube (liner) surrounded by at least an outer tube and/or a prestressed strip sheath, a pressure-generating punch insertable into the cylinder for generating a high pressure in a pressure medium enclosed in the pressure chamber, and a die insertable into the cylinder and having an opening with the cross-section which the extrusion product is to acquire. Between the inner tube (liner) of the cylinder and the pressure-generating punch and the die, respectively, there are seals, usually of the type containing a first sealing ring with a cylindrical surface making contact with the punch and the die, respectively, and an end surface making contact with a seal holder and a second sealing ring with an outer cylindrical surface making contact with the inner tube (liner) of the cylinder and an end surface making contact with said seal holder. Further the press comprises an operating device for inserting a billet into the high-pressure cylinder and for pushing in the pressure-generating punch into the cylinder for generating a pressure in a pressure medium surrounding the billet in the pressure chamber, which is required for extrusion, the billet material being pressed out through the die opening, thus obtaining a product having the desired cross-section. A press of the kind described above is described more fully in U.S. Pat. No. 3,620,059.
In hydrostatic extrusion the pressure in the pressure chamber is between 10 and 15 kbar, which means that the stresses in the cylinder included in the pressure chamber will be very high. Furthermore, the pressure varies between 1 bar and the said high value for each work cycle, which results in risks of fatigue in the cylinder material. The corners of the inner surface of the cylinder and its end surfaces are subjected to the highest and most dangerous stresses and are thus most critical from the point of view of fatigue. In order to increase the life of the inner tube (liner) a reduction of the stresses in this critical part is necessary. Moving the seal inwards so that the outer part of the cylinder is not subjected to a radial pressure from the pressure medium is favourable from the point of view of fatigue. It is not possible to obtain a satisfactory support for a seal inside the cylinder at a distance from the end surface at the high pressure used because of the increase in diameter of the cylinder when the pressure is raised to the extrusion level. In order to obtain a satisfactory support, a support for the seal must be used which makes contact with the end surface of the inner tube and which bridges the whole gap between the cylinder wall and the pressure-generating punch and the die, respectively, both in case of unloaded and loaded pressure-chamber cylinder, apart from the necessary minor clearances.